The reports come from the FDA/CDC Vaccine Adverse Event Reporting System (VAERS). It's the early-warning system designed to give the first signal of vaccine safety issues. But a VAERS signal does not necessarily mean that a vaccine is unsafe.

Febrile seizure is relatively common. About one in 25 kids under age 5 will have at least one febrile seizure, usually triggered by illnesses associated with a fever such as an ear infection, a cold, the flu, or measles. Febrile seizures triggered by vaccination are uncommon.

However, reports to the VAERs system by parents and doctors suggest that there have been more febrile seizures than usual among kids getting the Fluzone vaccine. Fluzone is the only vaccine approved for kids ages 6 to 23 months.

Investigating Cases of Febrile Seizure

"Data from VAERS are preliminary and serve as a sign or indication that further investigation is warranted," the FDA says in a news release posted on its web site. "Further investigations are under way to assess whether there could be an association between influenza vaccination and febrile seizures, or if other factors could be involved."

In a statement, Sanofi Pasteur says it is working closely with the FDA and "will thoroughly assess all cases of febrile seizure and any other adverse experiences reported following administration of our vaccines."

Sanofi notes that despite increased use of the Fluzone vaccine over past years, there has been no overall increase in febrile seizures.

An unusually stringent study of children vaccinated with the 2009 H1N1 pandemic flu vaccine found no link between the vaccine and febrile seizure.

Symptoms of Febrile Seizure

Children who have a fever are "febrile." Febrile seizures usually occur in kids with a fever of 102 degrees Fahrenheit or higher, and may also occur as a fever is going down.

Symptoms of febrile seizure are large or small spasms or jerking movement that usually last for only a minute or two. Although the child may briefly lose consciousness, febrile seizures do not cause permanent neurological damage.

About a third of kids who have one febrile seizure will have another one. However, fewer than 10% of kids who experience febrile seizures will turn out to have epilepsy.

The FDA says it will report any new information on Fluzone safety as soon as it becomes available.